A single battery cycle consists of bringing the battery up to a full (100%) state of charge (SOC) from particular end-of-discharge voltage. Partial cycles will affect a battery’s longevity, especially if the batteries are not fully recharged after each discharge. Batteries are capable of only a certain number of cycles before capacity loss becomes noticeable and the battery reaches the end of its life. Proper charging is critical to ensuring a battery reaches it fully rated life capacity.

There are two ways in which a battery is used—floating or cycling. A battery designed for backup will spend most of its time in float mode, which means it is under a constant, small charge to stay full and ready. Its life expectancy at a specific temperature (usually 77°F) is expressed in years. A deep-cycle battery used in off-grid RE applications, on the other hand, will have its life expectancy expressed in the number of cycles delivered to a specific depth of discharge (DOD). If a 100 Ah battery delivers 50 Ah on each cycle, it is discharged to a 50% DOD.

The graph shows a typical battery’s expected cycle life under various DODs. At 20% DOD, the battery will deliver 5,000 cycles; at 50% DOD, the same battery can deliver only 2,750 cycles before reaching the end of its life. A battery is considered to have reached its end of life when it fails to support the load for at least 50% of the designed time.